The present disclosure generally relates to a load limiting device for a vehicle safety restraint belt. More particularly, the present disclosure relates to a variable load limiting device for a seatbelt retractor.
Conventionally, a vehicle safety restraint comprises seat belt webbing which passes around the torso and around the lap of a vehicle occupant to secure the occupant in a vehicle seat. One end of the seat belt webbing is attached to a releasable buckle and the other end is wound onto a rotatable spool of a retractor. The retractor allows a vehicle occupant to move, provided the movement is relatively gentle. However, in a crash when the occupant is subject to a sudden deceleration, the retractor locks to prevent any further payout and to securely restrain the occupant.
In high velocity crashes, the force exerted by the seat belt webbing on the vehicle occupant is extremely high and can itself cause injury. Load limiting devices are generally provided to controllably absorb and dissipate some of this force felt by the occupant. The load limiting devices can be introduced into any part of the webbing force path. For example, a load limiter may be incorporated into the buckle mounting, or into the retractor itself, such as in the winding of the spool.
The load limiting device can have one or more components which become mechanically deformed when a pre-determined force is exceeded. Torsion bars are, for example, used as such deformable components and are arranged to twist in the direction of the belt winding out due to a rotational movement of the retractor.
A problem with the known systems which rely on bending torsion bars is that the force absorbed is the same at low speeds, with small, light occupants, as at high speeds with large occupants. This constant force means that the large occupant, or the high speed crash, causes more payout of seat belt webbing than for low speeds, and in worst case situations the forward movement of the occupant, despite the belt restraint, can be so excessive as to result in an impact with an interior part of the vehicle thereby causing injury.
Hydraulic systems have been used in which a feedback loop is provided so that a higher force can be absorbed with a higher speed of movement. In these arrangements, the absorbed force rises exponentially with the speed of movement of the occupant. However, this rapidly increasing force absorption can be too severe and at very high speeds the force felt by the occupant is likely to be excessive.